FIG. 21 is a schematic plan view of a print head 1 included in a known inkjet line printer.
In line printers, one line is simultaneously printed on a print medium. Accordingly, the print head 1 includes a plurality of print head chips 2 (2A, 2B, . . . ) which are arranged in a direction in which lines are printed. Although only five print head chips 2A to 2E are shown in FIG. 21, more print head chips 2 are actually arranged.
Although not shown in the figure, each print head chip 2 is constructed by, for example, disposing heating elements for heating ink on a semiconductor substrate, forming ink-pressurizing cells such that the ink-pressurizing cells surround their respective heating elements, and disposing a nozzle sheet having nozzles for ejecting ink drops above the heating elements. Ink contained in the ink-pressurizing cells is heated by rapidly heating the heating elements, and is ejected from the nozzles due to force applied by bubbles of ink vapor (ink bubbles).
In addition, the print head 1 is provided with an ink path 3 (region between the double-dotted chain lines in FIG. 21) which extends along the length of the print head 1. The ink path 3 is used for supplying ink to the ink-pressurizing cells of the print head chips 2. The print head chips 2 are arranged along the ink path 3 and are disposed on both sides of the ink path 3. In addition, the print head chips 2 on one side of the ink path 3 and the print head chips 2 on the other side face each other across the ink path 3. More specifically, the print head chips 2 on one side of the ink path 3 are rotated 180 degrees relative to the print head chips 2 on the other side. Accordingly, the ink-pressurizing cells of all of the print head chips 2 are communicating with the ink path 3.
In addition, in FIG. 21, the print head chips 2 are alternately disposed on the upper side and the lower side of the ink path 3 along the length of the ink path 3; that is, the print head chips 2 are arranged in a zigzag pattern.
More specifically, the print head chip 2A at the left end in FIG. 21 is placed on the upper side of the ink path 3, and the print head chip 2B, which is adjacent to the print head chip 2A, is placed on the lower side of the ink path 3 in FIG. 21. In addition, the print head chip 2C, which is adjacent to the print head chip 2B, is placed on the upper side of the ink path 3 in FIG. 21.
In addition, although not shown in the figure, the print head chips 2 are arranged such that if an interval between the adjacent nozzles in each print head chip 2 is L, an interval between the nozzles at the ends of the adjacent print head chips 2 (an interval in the direction in which the print head chips 2 are arranged) is also L. For example, in FIG. 21, an interval between the right end nozzle of the print head chip 2A and the left end nozzle of the print head chip 2B is L. Accordingly, even when ink is ejected from a plurality of print head chips 2, all ink drops land on the print medium at a constant interval L.
FIG. 22 is a sectional view of FIG. 21 cut along line A—A, and an ink-path member 4 placed on the print head chips 2 is also shown in FIG. 22. FIG. 23 is a sectional view of FIG. 21 cut along line B—B, and the ink-path member 4 is also shown in FIG. 23. FIG. 24 is a sectional view of FIG. 21 cut along line C—C, and the ink-path member 4 is also shown in FIG. 24.
As shown in FIGS. 22 to 24, the ink-path member 4 is placed on the top surfaces (surfaces facing the ink-path member 4) of the print head chips 2. The ink-path member 4 has a groove 4a (having a bracket shape in cross section) which extends along the length of the ink-path member 4 and which communicates with the ink path 3. In addition, the ink-path member 4 also has recesses 4b for receiving the print head chips 2 in the bottom surface thereof. The number of the recesses 4b provided is the same as the number of the print head chips 2, and the size of the recesses 4b is slightly larger than the size of the print head chips 2.
When the ink-path member 4 is placed on the print head chips 2, the groove 4a of the ink-path member 4 is positioned directly above the ink path 3 and the print head chips 2 are disposed in their respective recesses 4b. Then, the recesses 4b and the print head chips 2 are adhered to each other. The ink-path member 4 does not have the recesses 4b and is directly adhered to a nozzle sheet 5 at regions where the print head chips 2 are not disposed (see left side in FIG. 24). Accordingly, the spaces between the ink-path member 4 and the print head chips 2 and the spaces between the ink-path member 4 and the nozzle sheet 5 are sealed with an adhesive layer.
In the print head 1 which is constructed as described above, ink flows through the groove 4a of the ink-path member 4 and the ink path 3 and is supplied to the ink-pressurizing cells of each print head chip 2 without leaking out of the print head 1.
In the above-described known technique, there are certain limits to the processing accuracy of the print head chips 2, the positioning accuracy when the ink-path member 4 is adhered to the print head chips 2, and the processing accuracy of the recesses 4b of the ink-path member 4.
Accordingly, when the accuracy error exceeds a certain limit, there is a possibility that the spaces between the ink-path member 4 and the print head chips 2 cannot be completely sealed when the ink-path member 4 is adhered to the print head chips 2, and gaps will be generated between the ink-path member 4 and the print head chips 2. Accordingly, there is a risk in that ink will leak out of the print head 1 though these gaps.
FIGS. 25 and 26 are sectional views which correspond to FIGS. 22 and 24, respectively, showing the case in which the ink-path member 4 includes an error.
As shown in FIGS. 25 and 26, it is assumed that a surface 4c between the recesses 4b of the ink-path member 4 has an error and the amount of error of the surface 4c is X. In this case, when the ink-path member 4 is placed on the print head chips 2, the surface 4c of the ink-path member 4 first comes into contact with the nozzle sheet 5. At this time, the distances between the recesses 4b and the print head chips 2 are larger than the designed value by X, and gaps S are generated accordingly. Similarly, gaps S are also generated between the nozzle sheet 5 and the surfaces other than the surface 4c where the recesses 4b are not provided. If the gaps S are too large to be completely sealed with an adhesive, ink will leak out though the gaps S.
On the other hand, in the above-described known technique, heat is emitted from the print head chips when they are driven, that is, when the heating elements are heated, and there is a problem as to how the heat generated in the print head chips is to be dissipated.
A part of heat generated by the heating elements goes out along with ink when the ink is ejected, but the remaining heat accumulates in the print head chips. Accordingly, when ink is continuously ejected (when printing is continuously performed), a temperature increase of 100° C. or more occurs in a short time in the print head chips.
In particular, heat generation cannot be ignored in print heads for line printers since they include many print head chips and there are the same number of heat generators as the number of print head chips.
In order to properly eject ink, the operating temperature of the print head chips must not be higher than the boiling point of ink (approximately 100° C.). If the temperature exceeds this limit, a state in which a proper amount of ink is properly ejected cannot be obtained and the printing quality will be degraded.
Accordingly, a method is known in which when printing is performed for a predetermined time, the operation is stopped for a predetermined time interval to reduce the temperature before the operation is restarted. However, this method has a problem in that the overall print speed is reduced if the stopping time is increased to suppress the temperature increase.
Alternatively, a heat-dissipating member may be installed in the print head. However, in the case in which the heat-dissipating member is installed in the print head, sufficient ambient dissipation cannot be provided unless the surface area of the heat-dissipating member is large. Accordingly, there is a problem in that the size of the print head is increased if a large heat-dissipating member is installed. On the contrary, if the surface area of the heat-dissipating member is reduced, sufficient ambient dissipation cannot be provided.
In addition, print head chips are generally arranged in a zigzag pattern in known print heads for line printers, and it is difficult to accurately process the heat-dissipating member in accordance the arrangement of the print head chips and install it.